So, I had an idea for building a 3-way speaker the other day. I like the work Zaph has done on his website and it inspired me to try something with the HiVi B3S. I also have heard the Dayton 10" Hifi subs and love them. A friend is loaning me Seas 27TDFC tweeters to complete the setup. Does anyone see anything inherently wrong with this setup.

Well, I'm not sure I'd advocate following that project - the B3S line array + single tweeter is actually a pretty bad idea for several reasons.
First, point source bass + line array mid + point source tweeter is a recipe for disaster. Your on-axis spectral balance will only be right at one point, and your power response will be messed up. Plus, the c-t-c spacing requires rolling the B3S off early, once again not really taking advantage of it's full-range strength.

I like the B3 drivers (I have the N which looks nicer), but a line of 4 is problematic - it's not really a line array, but not really a point source either. This isn't to say it won't work, but IMHO spending another $10 per side and using a WR125s as a mid instead of the 4 HiVi's would be a much easier path to success.

If you *really* want to use 4 HiVi's, another idea would be a bizarre MTM setup with two HiVi's side-by-side making the mid. ie

MM
T
MM

I think this would have somewhat smoother power response overall than the line-of-4, and you wouldn't have any horizontal lobing problems between the M and the T.

On a totally seperate note, I like the idea of 2.5 way setups. I dont want to try MTM for my first speaker and 2-way is just too BORING for me. I know there are lots of designs available for this. Since I already have the 27TDFC I'll need to look for a driver to mate up with it but with solid low end so i can use it as a bass driver as well. I want a low distortion setup but not willing to pay $$$ for Seas Excel or SS mids.